The present invention relates to organic electroluminescent (EL) devices and, more particularly, to forming preformed images in such devices.
Images are commonly printed on paper, fabric, ceramics and other substrate media that provide a mechanical and reflective/transmissive substrate for the handling and viewing of such images. These substrates often include a reflective surface as background that allow direct viewing of the images using ambient light or other forms of applied illumination. Transparency media such as slides or translucent display media require a lit backing or projecting system that illuminate the transparent media to either view directly (displays) or to project onto screens (slides). The use of the type of medium depends on the specific application, e.g. consumer images for home display, commercial billboards, lit advertising displays, and many others.
For either type of application, current media suffer a number of drawbacks. Reflective media require ambient illumination for viewing; a darkened living room for example, may not provide sufficient illumination levels for enjoyable viewing. Transparency media, on the other hand require the expense and additional bulk of light sources, view boxes, and associated subsystems.
For projection displays, supplemental screens, darkened rooms, and optical projection systems are normally required. Flat panel display technology has provided in some cases solutions to the illumination and bulk problem. Devices such as Liquid Crystal Displays (LCDs) are electronically controlled transparencies that when back illuminated with flat profile light sources provide an integrated and reasonably compact system to view pictorial and graphic content. Unfortunately, LCDs have problems with brightness, angle of viewing, and- although superior to the illuminated view box- are still bulky relative to a standard print on paper. Organic Light-Emitting Devices (OLEDs), also called organic electroluminescent (EL) devices, are self-illuminated display media that do not require supplementary illumination sources and hence are advantaged in compactness and power consumption. OLEDs are also Lambertian emitters and do not exhibit significant angle viewing problems. U.S. Pat. Nos. 4,356,429 and 4,539,507 describe OLED device configurations and materials. In order to depict pictorial and other types of content on LCDs, OLEDs and other flat panel display devices, x-y addressable electronic subsystems are required to apply controlled voltages across the display medium that are representative of the image being displayed. The x-y addressing voltage in the case of OLEDs, for example, controls the amount of current that flows through an organic light-emitting structure that produces light of color and brightness controlled by the chemical and physical characteristics of said layer. A multiplicity of colors can be achieved using OLED technology by chemical manipulation of the layers as taught in U.S. Pat. No. 4,769,292, for example. x-y image-wise address of the emissive layers can be achieved using a number of approaches. In the so-called passive matrix device orthogonal sets of electrodes are sequentially addressed in a line-by-line manner with voltage levels that correspond to the spatial image brightness desired at each specific position of the display device. In the active matrix configuration, a set of defined transistors usually defined onto single crystal silicon or glass (thin film transistors) are addressed and apply specific voltages across the display medium to control the brightness or transmissivity of the display medium.
There are many applications where it is desired to have preformed images in a display device. Although pixelated flat panel displays solve the illumination and bulk problem of traditional media, their cost is relatively high when providing preformed images and this is due primarily to the electronic interconnections and driving circuitry required. Flat panel displays are also limited to relatively small sizes due to the semiconductor processes used to fabricate them and cost issues that scale with display area. Although more flexible in their ability to display more selectable and changeable content than their traditional counterparts, cost and size issues make substitution of standard display applications using pixelated flat panel displays impractical when producing preformed images.
It is therefore an object of the present invention to effectively form preformed images in an organic electroluminescent (EL) device.
This object is achieved by a method of making an organic EL device with a preformed image, comprising the steps of:
a) providing a light-transmissive substrate;
b) providing a light-transmissive anode electrode over the substrate;
c) forming an organic EL medium structure over the anode electrode;
d) forming a cathode electrode over the organic EL medium structure and bonding a cover structure over the device; and
f) using a laser beam to remove cathode material from the cathode electrode to form the preformed image, such that when a voltage is applied between the anode and cathode electrodes, light will not be emitted in areas corresponding to the removed cathode material and the preformed image will be observable by a viewer through the light-transmissive substrate.
It is an advantage of the present invention that by removing cathode material in an organic EL display device, preformed images can be effectively formed and displayed. The device can be fabricated and fully covered or encapsulated and then the images can be formed by removing cathode material.
The present invention does not require complex wet chemistry processing to form preformed images in organic EL display devices. Moreover, complex masks and shadow masks are not required.